1. Field of the Invention:
This invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating gate oxide, which improves the electrical quality of a gate oxide layer.
2. Description of Related Art:
Since gate oxide is one of the important components in a metal-oxide-semiconductor (MOS) device, the electrical quality of the gate oxide directly affects the quality of a MOS device. Conventionally, a wet oxidation process is usually performed on a gate oxide layer that exceeds 200 .ANG. in thickness. In a current very-large-semiconductor-integration (VLSI) process, the gate oxide layer used in a MOS device, which has a thickness of about 100-200 .ANG., is normally formed by a dry oxidation process in the presence of C.sub.2 H.sub.2 Cl.sub.2 vapor.
If the required thickness of gate oxide is thinner than 100 .ANG., perhaps even as thin as about 50 .ANG., such as the tunneling oxide within a flash memory, the method for fabricating gate oxide is normally a dry oxidation. However, if the required thickness of the gate oxide is thinner than 50 .ANG., it is very difficult to obtain a high-quality gate oxide layer through a conventional oxidation process.
As shown in FIG. 1, which is a flowchart represents a conventional method for fabricating gate oxide, an oxidation process is performed on a provided silicon substrate in step 10. A gate oxide layer is formed on the silicon substrate by performing a thermal oxidation in an environment filled with oxygen at a temperature of about 900.degree.-100.degree. C. Then, in step 20, an annealing process is performed in an environment filled with gas, such as nitrogen or inert gases, at a temperature of about 900.degree.-1100.degree. C. This eliminates the stress within the gate oxide layer caused by defects.
A gate oxide layer with a thickness of about 40 .ANG. formed by the foregoing method has a measured breakdown charge (Q.sub.bd) equal to or less than 5 coulombs per square centimeter. A measured leakage current through the gate oxide layer is about 34.6.times.10.sup.-8 amperes under a 3.2-volt gate voltage. This shows that the electrical quality of the thin gate oxide is not acceptable.